The invention relates to a semiconductor device comprising a semiconductor body having at least a bipolar transistor with an emitter zone of a first conductivity type adjoining a surface of the body, a likewise surface-adjoining base zone of the second conductivity type which within the body surrounds the emitter zone entirely, and a surface-adjoining collector zone of the first conductivity type. The base zone comprises an active base region and a base contact region which is associated therewith and which is deeper and more highly doped than the active base region. The base zone, like the emitter zone, is contacted at the surface and the collector zone comprises a surface-adjoining collector contact region of the first conductivity type which is contacted at the surface and has a higher doping concentration than the adjoining semiconductor material. Viewed in a direction parallel to the surface, the active base region is present between the base contact region and the collector contact region, the emitter zone, the active base region, the base contact region and the collector contact region are provided in and have a different doping than a surface-adjoining semiconductor layer that is substantially homogeneously doped and that surrounds the collector contact region and the base regions.
The invention relates in addition to a particularly suitable method of manufacturing such a semi-conductor device.
A semiconductor device of the abovedescribed kind is known, for example, from the U.S. Pat. No. 3,766,446.
In semiconductor technology, and in particular in monolithic integrated circuit technology, it is often endeavoured to produce circuits, and hence semi-conductor circuits, which can be used up to very high frequencies, for example, up to frequencies of one or a few Gigahertz (GHz). In addition it is in many cases desirable for a monolithic integrated circuit to comprise bipolar transistors of both the npn-type and of the pnp-type.
Although reaching such very high frequencies presents technological problems already for vertical npn-transistors, this is the case in particular in monolithic circuits having npn-transistors and pnp-transistors in a single epitaxial layer. The pnp-transistors are nearly always constructed as lateral transistors. Therefore, it is not only substantially impossible to make the pnp-transistors suitable for very high frequencies, due to the lateral structure and due to the low hole mobility, but in general the npn-transistors and pnp-transistors provided in a single epitaxial layer in this manner will show electrically important differences due to their greatly differing geometric structure, which in general is not desirable.
It has been endeavoured to solve this problem by giving both the npn-transistors and pnp-transistors a vertical structure, while using two or more epitaxial layers present one on top of the other, but in addition to the fact that the provision of several epitaxial layers results in a considerable technological complication, further problems occur due to the out-diffusion of the buried layers present at different levels.
In the described known transistor structure these problems occur to a far smaller extent, but an important drawback is that in the known transistor according to the U.S. Pat. No. 3,766,446, the emitter zone is present for a large part within the highly doped base contact region. The emitter-base junction of the known transistor thus comprises a considerable part across which substantially there occurs no injection of minority charge carriers into the base, but which due to its extra surface, does increase the emitter-base capacitance considerably, to which the high doping of the base contact region contributes additionally. In particular at low currents this has a very adverse influence on the high-frequency characteristics, such as inter alia the cut-off frequency.